(1) Field of the Invention
The present invention relates to an apparatus for counting the number of record tracks formed on an optical disc. The present invention relates particularly to the apparatus therefor applicable to an access control system used in a sample servo type optical disc apparatus.
(2) Background of the Art
Successive pit servo type and sample servo type tracking methods applied to, for example, a tracking read/write type optical disc have been proposed.
Optically and magneto-optically recordable discs are known. One has a preformed continous groove as shown in JP-A-52-10102, published on Jan. 26,1977, the other has preformed pits as shown in U.S. Pat. Nos. 4,402,061 and 4,561,082. A tracking servo is carried out for the preformed continous groove or preformed pits on the disc. The former we call a continous servo type, the latter a sample servo type, hereinafter.
In a previously proposed optical disc using the sample servo method, an optical head is accessed onto a desired record track having a prerecorded address. At this time, an optical head access driving portion including, e.g., a thread motor is driven in response to an access control signal CONT supplied from a system controller, so that the optical head traverses the record tracks.
During the transverse operation of the optical head, a pick-up signal PU derived from the optical head is inputted to a pick-up signal processing circuit. In addition, a tracking information S.sub.TRK is outputted from the pick-up signal processing circuit. The tracking information S.sub.TRK is periodically derived when the optical head traverses one of the record tracks is inputted into a track counting apparatus and the optical head access driving portion is driven until the counted contents of the track counting apparatus coincides with one of the tracks to be accessed.
In the case of the sample servo type optical disc, the record tracks TR (- - - TR.sub.n-1, TR.sub.n,TR.sub.n+1, - - -) formed on the optical disc have a servo area ERP and data recording area ERD, which are aligned in a radial direction and, are alternatingly conjoined in a sequential order on each same track. Therefore, the data recorded on the data recording area ERD do not interfere with the tracking information and clocking information recorded on the servo area.
A pair of tracking pits PA, PB preformed in the servo area ERP are offset toward an inside and outside of a center line of the above-described record tracks by a 1/4 (one-fourth) pitch with respect to the center line CET of the above-described record tracks. In addition, a pit PC for a clocking signal is formed on the center line CET.
When a read light beam LAR the center line CET so that the light beam impinges sequentially on the pits PA, PB, and PC, the optical head can detect the three pits PA, PB, and PC as the change of reflected light beam from these pits.
The pick-up signal derived at the optical head on the basis of the reflected light beam from the disc is sampled and held (sample/hold) by means of a sampling pulse generated by the pick-up signal processing circuit at a timing at which the read light beam LAR passes through the pits PA, PB, and PC. The pick up signal processing circuit can produce the tracking information representing a positional difference between the read light beam LAR and center line CET and produce clock information representing a timing at which the read light beam LAR has passed on the clocking signal pit PC.
The system controller carries out a tracking control for the optical head so that the read light beam LAR scans the center line CET of the record tracks - - - TR.sub.n-1, TR.sub.n, TR.sub.n-1, - - - on the basis of the tracking information derived in the pick-up signal processing circuit. The pick-up signal processing circuit generates a sampling pulse which is synchronized with the phase of the clocking information, so that the magnetically-optio recorded data on the data recording area ERD is sampled from the pick-up signal PU in accordance with the sampling pulse.
The track counting apparatus counts the number of tracks which the read light beam has traversed on the basis of the tracking information S.sub.TRK transmitted from the pick-up signal processing circuit during the head access operation.
In detail, when the read light beam LAR traverses the record tracks TR sequentially, a track transverse information INF.sub.TR sampled by means of the sampling pulse signal P.sub.s at a timing of the tracking signal pits PA and PB exhibits a change corresponding to one period of a sinusoidal wave during the time when the read light beam LAR is moved from one record track TR.sub.n to the other track TR.sub.n-1 (or TR.sub.n+1) at the outside (or inside) adjacent to the track TR.sub.n.
Since the pair of tracking pits PA and PB are formed in a discrete configuration with the data recording area ERD sandwiched, a track transverse information INF.sub.TR constituted by a stepwise, sinusoidal waveform is obtained. The track transverse information INF.sub.TR is passed through a filter so that a clear phantom track transverse information INF.sub.TRX is derived. Therefore, the track counting apparatus can count the number of record tracks which the read light beam LAR has traversed by counting the number of times the track transverse information INF.sub.TR traverses a predetermined threshold level SHD on the basis of the change of the sampled track transverse information I.sub.TR.
If a transverse speed of the head becomes increased when the counting apparatus counts the number of the record tracks which the light beam LAR has traversed on the basis of the track information INF.sub.TR, it may become impossible for the track counting apparatus to accurately count the number of tracks which the read light beam has traversed.
In detail, the optical head gradually accelerates from a state in which the speed is zero during an accelerated transverse interval of time T.sub.1 immediately after the optical head has started, until the optical head accesses at a high speed one of the other tracks from a state during which the head is tracking one of the record tracks. Therefore, a period of the phantom track transverse information INF.sub.TRX (hence, the track transverse information INF.sub.TRX is gradually reduced until the head enters a high-speed transverse interval at which the head moves at a high velocity.
On the contrary, since the sampling period of the sampling pulse used in the pick-up signal processing circuit is constant, the number of samplings per period of the track transverse interval information INF.sub.TR at an interval of the accelerated transverse T.sub.1 of the head are gradually reduced. Thereafter, at the high-speed transverse interval, a time axis is expanded so that the sampling period .alpha. becomes substantially equal to one period .alpha..sub.TRX of the phantom track transverse information INF.sub.TRX. Consequently, a value of the track transverse information INF.sub.TR enters in a state in which, for example, only several numbers of sampling information can be extracted during one period of the above-described sampling.
If the transverse speed is further increased and the period .tau..sub.TRX of the phantom track transverse information INF.sub.TRX becomes shorter than the sampling period .tau..sub.SP of the sampling pulse P.sub.S, only one sampling information can be extracted for several periods of the phantom track transverse information INF.sub.TRX as the track transverse information INF.sub.TR.
In such a state as described above, the track transverse information cannot be reproduced having the same contents as the phantom track transverse information INF.sub.TR before the sampling from the track transverse information INF.sub.TR. Hence, even when the track transverse information INF.sub.TR exceeds the predetermined threshold level SHD, the reliability that the track transverse information INF.sub.TR represents the correct number of the traversed tracks becomes reduced.
Suppose that the above-described state has a relationship of a trajectory LARX of the read light beam LAR with respect to the record tracks TR. For example, the read light beam LAR has traversed five record tracks TR.sub.n to TR.sub.n+4 while the trajectory LARX is passing through the data recording area ERD between the mutually adjoining servo areas ERP. In this case, only one sampling information can be obtained while the phantom track transverse information INF.sub.TRX changes by five periods.
Information corresponding to the phantom track transverse information INF.sub.TRX cannot be reproduced from this one sampling information.
As a matter of fact, if the transverse speed of the read light beam LAR becomes so high that a theorem of sampling cannot be satisfied, the number of the head traversed tracks cannot be counted in the previously proposed method described above
Especially, in a case where the record tracks are sampled at a sampling frequency of 41 kHz for the representative optical disc having 1.5 micrometers of a track pitch and the read light beam LAR has traversed at a speed such as to generate the phantom track transverse information INF.sub.TRX, the read light beam LAR traverses the record tracks TR at a speed of 3 cm/sec. If the read light beam LAR traverses at a higher speed, the transverse track information cannot be reproduced in terms of the sampling theorem. Therefore, the information access speed from the optical disc is limited at a relatively slow speed value.